The present invention relates generally to generation of electrical power in stationary power plants. More specifically, the present invention provides a method and apparatus for production of electrical power with improved efficiency and/or improved capture of carbon dioxide.
Conventional methods of generation of electrical power in stationary power plants involves generation of high-pressure steam, and feeding of the steam to a steam turbine in which the steam pressure drops and the steam drives the turbine to generate electrical power. For example, a fossil-fuel fired boiler may be used to produce high-pressure superheated steam that is fed to an upstream steam turbine in a series train of turbines in which steam flows from higher pressure turbines to lower pressure turbines, all of which are connected to a drive shaft of an electrical generator for producing electrical power. Optionally the steam may be taken at an intermediate pressure and superheated in the fossil fuel fired boiler before returning to the train of steam turbines for further expansion.
Many existing power plants include coal-fired boilers, and use of coal-fired boilers is desirable in that a significant amount of the world's fossil fuel reserves exists as coal. Undesirably, CO2 emissions per kWh of electricity production from a coal-fired power station are relatively high, and may be more than double that from a natural gas fuelled combined cycle power station. Further, CO2 capture efforts to reduce undesirable CO2 emissions reduce the efficiency of power plants and lead to higher power generation costs.
The amount of electrical power produced is a function of the temperature and pressure steam conditions of steam fed to the turbines. Increased temperature and pressure of the steam tends to increase electrical power production, and thus overall power production efficiency. However, the materials, etc. of turbines and boilers provide effective limits to the steam conditions for a given power plant. Although higher performance turbines and boilers may be available, the costs of such higher performance equipment are often excessive, particularly for high performance boilers. Additionally, there may be other problems in using such high-performance equipment. For example, for a conventional coal-fired steam boiler, there may be an operating limit of 600° C. at 300 bar pressure. To obtain significantly higher temperatures and pressures may require very costly super-alloys that are high in nickel content, which may result in problems from ash and slag deposition and corrosion. These alloys are not currently approved for use in steam boilers but they can be used in heat exchangers for superheating of steam. Accordingly, the boiler and/or turbine have operating limits that limit as a practical matter the amount of increased power production efficiency that may be obtained by increasing the temperature and pressure of the steam.
What is needed are a method and apparatus for obtaining steam at higher temperature and pressure (and thus increased power generation efficiency) without the need for expensive high-performance boilers, and for reducing CO2 emissions per kWh of electricity produced, particularly for coal-fired power plants.